Rubber tree and latex production is of primary importance in SE Asia, and in Thailand in particular, as it represents an important source of income for the local population. It is believed that rubber tree plantations are often associated with a decrease in soil fertility, however, the real impact on the soil compartment is still largely unknown. In particular, the role of the soil biota, in relation to soil characteristics and soil functions, is still poorly understood. Soils in NE Thailand are mainly sandy and contain very low concentrations of nutrients and organic matter, and are considered unsuitable for the cultivation of many crops. As rubber trees require a moderate level of soil fertility and are able to grow in a wide range of environments (including poor sandy soils), plantations are expanding widely in NE
Thailand at the expense of other crops, such as cassava. In order to better manage and sustain soil fertility in this region, alternative management practices have been widely promoted, without strong scientific evidence of their effect on the soils. One such management strategy that has received increased attention in recent decades is the application of biochar. Biochar is the result of the pyrolysis of biomass under a low- or zero-oxygen environment, and is being used as a means for sequestrating carbon into soils while producing secondary agronomic benefits and improving soil functions. However, few studies focused on the long-term effect of biochar into soils and there is, to our knowledge, no such study on rubber tree plantations.
A better understanding of the role of the soil biological compartment in managing soil fertility in rubber tree plantations is thus mandatory. In this study, the soil bacterial and fungal
communities were studied along a chronosequence of 3, 6 and 16 year-old rubber tree plantations and compared to those of nearby cassava fields. Arbuscular mycorrhizal fungi (AMF) communities were also included in the study. To assess the effect of biochar
application on the soil bacterial and fungal communities associated with rubber trees, an experiment was set up in a 7 year-old plantation and included four doses of biochar (0, 5, 10 and 20 tons/ha). Both bacterial and fungal communities were characterized 18 and 28 months after biochar application while taking fine-scale soil variability into consideration.
Our results showed that microbial community structure and diversity were more strongly affected by the age of the plantation itself than by the conversion of cassava fields into rubber tree plantations. The opposite trend was observed for the functional microbial diversity, which was more affected by the conversion of the cassava fields than by the age of the trees.
Changes were particularly found in functions linked to the use of various carbon sources by soil bacteria, while functions linked to the N cycles remained unaffected. Fungal community was not affected by either the conversion from cassava or the age of the trees. While AMF community composition gradually shifted with the age of the plantation, the richness and the intensity of colonization remained high in all the studied sites. Changes in microbial
communities were strongly related to soil physical and chemical characteristics. pH was the main driver of the changes in the bacterial community, while soil P content was more related to the AMF community changes.
This study provided the first results on the microbial communities associated with rubber trees of different ages in poor tropical soils. Despite the fact that rubber tree plantations may not be as detrimental to the soil fertility as may have been expected, our results highlight the need for complimentary research to better understand the underlying mechanisms and the potential roles of the native microbial communities in managing soil fertility of perennial plantations in
differently affected the microbial community with regards to the soil class, both after 18 and 28 months. These results highlight the importance of the soil variability, even at the fine- scale. Fungal communities were generally more severely affected than bacterial communities and no dose-effect from biochar was observed in any of the soil classes and changes were related with changes to the chemical characteristics and the water status of the soils. Although biochar is generally considered as either neutral or beneficial with regards to soil fertility, more research is needed to ensure that the changes observed in the microbial communities are not responsible for changes in soil functions or other ecosystem services.
Results from such research may be of benefit to the national and local authorities that are responsible for plantation management policies. Appropriate use of alternative management practices, such as biochar addition, could thus be promoted with regards to the local
environmental conditions of the plantations. This would assist in the maintenance and restoration of soil fertility while limiting the use of mineral fertilizers that are expensive for the local growers.
Subsequent studies should focus on improving the understanding of the underlying
mechanisms of the relationships between soil microbial communities and other ecosystem services, including tree nutrition and latex production, both under conventional management and after the application of biochar. Other types of biochar, together with various application strategies (dose, depth, location, period and frequency of applications…) may also result in different results and this deserves further investigation. Since bacteria and fungi are important elements of complex soil food webs, other groups of organisms (including nematodes and other higher trophic levels of fauna) may also be significantly affected and result in substantial changes in soil functioning, but this has been poorly investigated to date. The durability of the changes is also yet to be assessed. The impact of several rotations of rubber
trees on the same site, for instance, is unknown. In the case of biochar addition, as its
residence time in soils is very long (hundreds to thousands of years), even small changes may result in long-lasting or irreversible consequences for soil functions and services but there is no information to answer to this question at this stage. In addition, biochar was shown to result in sometimes opposite effects with regards to the soil class, and more research is also needed to better understand the relationships between the effect of the biochar and the soil class (with different layering, having a direct effect on the water behavior in the different soil layers) and soil properties in general before its widespread application in a range of soils and conditions.
It is important that we better understand the role of the microbial communities in relation to other aspects of rubber tree plantation management and in different soil types to ensure that suitable and sustainable management practices are promoted in order to maintain and possibly restore the soil fertility of strongly depleted soils in tropical regions. More sustainable rubber tree management practices would also benefit the local populations through a reduced use of expensive inputs (mineral fertilizers in particular) and an increased source of income (higher latex yield).
The approaches developed in our study may be also useful in improving the management of other perennial plantations in the region, including oil palm and coffee tree plantations. Indeed, the roles of the microbial compartment in soil fertility in these agricultural systems are poorly understood and this deserves deeper investigation. As with rubber trees, sustainable economic benefits may result from improved management practices through the restoration of
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